Masticatory muscles develop specialized molecular and physiological properties that are required for their proper function. This specialization can be altered by pathology, including some types of temporomandibular disorders. The experiments proposed here are designed to advance our understanding of how these unique muscles form during development and maintain their specialized functions during adult life. By birth, developing muscle fibers are innervated and have begun to diversify into specialized fiber types. The masseter muscle, in particular, has a unique fiber type composition and myosin isoform content Muscle-specific gene expression is controlled in large part by four muscle-specific transcription factors -- MyoD, myogenin, Myf- 5, and MRF4 -- that are collectively termed muscle regulatory factors (MRFs). Each MRF is expressed in a distinct pattern during myogenesis. k addition, our preliminary studies of myogenin promoter- MRF4 transgenic mice suggest that early expression of MRF4 accelerates myofiber formation. Our hypothesis is that distinct temporal and spatial patterns of MRF expression may underlie muscle cell specialization in masticatory muscles such as the masseter, temporalis, and pterygoid. The experiments test this hypothesis by determining how targeted changes in MRF expression affect masticatory muscle phenotype. The specific aims are to (i) Use our myogenin promoter-MRF4 transgenic mice to determine how MRF4 controls maturation and fiber type diversification in masticatory muscles, (ii) Identify functions in masticatory muscles that are unique to Myf-5 or MRF4, by comparing masticatory muscle development in myogenin promoter-Myf-5 and -MRF4 mice, (iii) Identify myogenic functions in masticatory muscles that are unique to myogenin, by analyzing myogenin (-/-) mice which also carry myogenin promoter-MRF transgenes, and (iv) Determine how individual MRFs control repair of masticatory muscles by analyzing masticatory muscle regeneration in appropriate transgenic and knock-out mice.